1. Field of the Invention
This invention relates to thermoplastic polyamide compositions, and more particularly to such compositions having improved ductility or toughness, and to the processes for preparing such compositions.
2. Description of the Prior Art
Unmodified thermoplastic polyamides are generally regarded as "tough". For example, the polyamides have good elongation; high energy to break, as demonstrated in tensile tests; high tensile impact strength and high energy absorption as demonstrated in falling dart tests, e.g., the Gardner impact test. In one aspect of toughness the polyamide materials are quite deficient; namely, resistance to crack propagation. This deficiency is reflected in notch sensitivity, brittle breaks and occasional catastrophic failure of molded or extruded parts. The tendency of polyamides to break in a brittle rather than ductile fashion is a significant limitation of utility. A resin may be characterized in its tendency toward ductility by the notched Izod test ASTM D-256-56. With the normal notch radius of 10 mils, polyhexamethylene adipamide (66 nylon), dry as molded, will have a notched Izod value of about 1 ft. lb./inch of notch.
There is much prior art concerned with improving the impact strength of polyamides. A variety of additives have been added to polyamides with some improvement in toughness being obtained. British Pat. No. 998,439, for example, discloses a thermoplastic composition comprising a mixture of 50 to 99 percent linear polyamide and 1 to 50 percent of olefin copolymer particles, the olefin copolymer containing from 0.1 to 10 mole percent of acid groups. Many olefin copolymers are disclosed, but it is not required that the olefin copolymers have a tensile modulus of 20,000 or less. The dry as molded notch sensitivity increases to 4.6 ft. lbs./inch as the copolymer is increased to 40 percent by weight as described in Example 1.
Murch U.S. Pat. No. 3,845,163 discloses blends of 60 to 85 percent by weight polyamide and an acid-containing olefin polymer in which the acid is derived from an .alpha.,.beta.-ethylenically unsaturated carboxylic acid and in which at least 10 percent of the acid groups have been neutralized with metal ions. Murch is concerned primarily with weld-line toughness which does not have a specific relation with blend toughness. The Murch blend, however, also demonstrates improvement over the composition of British Pat. No. 998,439 with respect to blend toughness. Murch did not recognize that improved blend toughness of polyamide compositions can be achieved at lower concentration levels of polymer addition provided that at least one polymer present has a tensile modulus of 20,000 or less and the ratio of the tensile modulus of the polyamide to the tensile modulus of said polymer is greater than 10 to 1.
Two U.S. Patents, Kray et al. Nos. 3,388,186 and Seven et al. 3,465,059 disclose polyamide compositions which possess high impact strength, some values being greater than 10 ft. lbs./inch. The disclosed compositions are graft copolymers prepared from an ethylene containing copolymer. The disclosed copolymers do not contain sites which adhere to the polyamide through sites of the polyamide. There is also no recognition that the tensile modulus of the ethylene containing copolymer is no greater than 50,000 p.s.i. or that particle size is important. Furthermore, the Izod impact strength is determined with samples held at 50 percent relative humidity for 3 days prior to testing as disclosed in U.S. Pat. No. 3,388,186. In some compositions moisture causes sharp increases in the notched Izod value. This is shown in Table 1, page 6 of British Pat. No. 998,439.
Owens et al. U.S. Patent 3,668,274 teaches modestly improved impact strength of polycarbonamides modified with (A) a first elastomer phase of copolymers or terpolymers and (B) a final rigid phase thermoplastic stage containing amine-reactive moieties, preferably carboxylic acid groups. The soft modifier is coated with a rigid layer thus negating a large improvement in polyamide toughness that could be achieved with a copolymer modifier.